Apparatus for measuring the concentration of dissolved oxygen in an aqueous solution

ABSTRACT

A WATER-ABSORBENT OXYGEN-PERMEABLE MEMBRANE FOR EXAMPLE OF POLYETHYLENE, IS ARRANGED IN CONTACT WITH TWO ELECTRODES, FOR EXAMPLE OF SILVER AND LEAD REPECTIVELY, SO THAT AN ELECTROCHEMICAL CELL WILL BE FORMED WHEN WATER WITH AN APPROPRIATE ION CONTENT IS ABSORBED BY THE MEM-   BRANE. IN USE THE CHARACTERISTICS OF THE CELL ARE EFFECTED BY OXYGEN DIFFUSING INTO THE MEMBRANE FROM A SOLUTION UNDER INVESTIGATION.

Oct. 24, 1972 N, MaLEOD ET AL v 3,700,580

APPARATUS Fon NEAsUR-ING .THE CONCENTRATION 0F y mssoLvEn OXYGEN IN ANAQuEous soLurIoN Filed Aug. 1o. 1970 .Y `kx United States Patent O M3,700,580 APPARATUS FOR MEASURING THE CONCEN- TRATION OF DISSOLVEDOXYGEN 1N AN AQUEOUS SOLUTION Norman MacLeod, Edinburgh, Scotland, andDavid Proctor, Highett, Victoria, Australia, assignors to NationalResearch Development Corporation, London, England Filed Ang. 10, 1970,Ser. No. 62,502 Claims priority, application Great Britain, Aug. 11,1969, 40,040/69 Int. Cl. G01n 27/46 U.S. Cl. 204-195 P 6 ClailnsABSTRACT F THE DISCLOSURE A water-absorbent oxygen-permeable membranefor example of polyethylene, is arranged in contact with two electrodes,for example of silver and lead respectively, so that an electrochemicalcell will be formed when water with an appropriate ion content isabsorbed by the membrane. In use the characteristics of the cell areaffected by oxygen dillusing into the membrane from a solution underinvestigation.

This invention relates to apparatus for measuring the concentration ofdissolved oxygen in an aqueous solution.

A known form of apparatus of this kind comprises a pair of electrodes,an electrolytic medium in contact with both electrodes so as to form anelectrochemical cell (which may be a voltaic cell or a polarographiccell), and a membrane which is permeable to oxygen and is arranged withone face in contact with said medium and the other face exposed forcontact with a solution to be investigated; in the operation of thisform of apparatus, a solution under investigation is separated from thecell by the membrane, through which oxygen diffuses into the cell so asto affect its characteristics to an extent dependent on theconcentration of oxygen in the solution. With this known form ofapparatus the response to changes in oxygen concentration in a solutionunder investigation is usually relatively slow, and it is accordingly an'object of the invention to provide apparatus of the kind specifiedwhich is improved in this respect.

In accordance with the invention the known form of apparatus referred toabove is modified by omitting the conventional electrolytic medium, bymaking the membrane of a material which absorbs water (at least when inthe form of a thin film), and by arranging the membrane in Contact withboth electrodes.

Thus an apparatus according to the invention comprises a pair ofelectrodes spaced from each other and a waterabsorbent oxygen-permeablemembrane having one face in contact with both electrodes and the otherface exposed for contact with a solution to be investigated.

With such an arrangement the apparatus may be activated for use bybringing the membrane into contact for a sufiicient time with an aqueoussolution containing a concentration of hydroxyl ions such that anelectrochemical cell will be constituted by virtue of the water absorbedby the membrane; it will often be found that solutions of the kind whichare commonly investigated using such an apparatus, for examplebiological uids and polluted waters, contain an adequate concentrationof hydroxyl ions for this purpose, in which case the apparatus maysimply be `activated by means of a solution of the kind which is to beinvestigated. In may, however, be preferred to provide within theapparatus itself a source of hydroxyl ions which will diffuse into themembrane when it absorbs water, for example in the form of crystals of aICC suitable substance such as ammonium sulphate disposed in contactwith the membrane; in this case the apparatus may be activated for useby bringing the membrane into contact for a sufiicient time with water(which may be pure) in either the liquid or the vapour state, the latternormally being preferred because a shorter time is required to completethe activation.

In use the characteristics of the cell thus constituted will be affectedby oxygen diffusing into the membrane, and it will be appreciated thatthe distance the oxygen has to travel before taking effect may be maderelatively small, so as to ensure a rapid response to changes in oxygenconcentration in a solution under investigation; by making the membranesuiciently thin, it is possible to achieve response times of the orderof 0.1-1 second. In the case where no internal source of ions isprovided in the apparatus, it is found that the characteristics of thecell are ,also markedly dependent on the pH value of the solution inwhich the apparatus is used; in this case, therefore, the use of theapparatus must be restricted, if reliable results are to be obtained, tosolutions whose pH values are substantially invariant. The sensitivityto the pH value of the solution is much less in the case where aninternal source of ions is provided, and can be reduced to insignifcancein this case by increasing the thickness of the membrane (with aconsequent sacrifice in respect of the response time and the ease ofactivation of the apparatus). In both cases, of course, if absolutemeasurements of oxygen concentration are required the apparatus mustfirst be calibrated using a standardised solution.

In order to ensure intimate contact between the membrane and theelectrodes, and thereby preclude the possibility of a lilm of waterbeing formed on the electrode side of the membrane when the apparatus isactivated, it is preferred that the membrane should be formed in situ bya suitable coating process. The membrane may suitably be of a polymericmaterial which absorbs water at least when in the form of a thin film, aspecific example being polyethylene. With this material the membrane mayconveniently be formed in situ by the application of a solution ofpolyethylene in a suitable solvent such as toluene; the thickness of theresultant membrane can readily be controlled by selection of theproportions of the solvent. Having regard to the factorsdiscussed above,a suitable value for the thickness of the membrane will normally be ofthe order of 0.001 inch.

The electrodes may suitably be of silver and lead respectively beingadapted respectively to constitute the cathode and anode of a voltaiccell completed by the electrolyte held in the membrane; with thisarrangement the output of, or the maximum current that may be drawnfrom, the cell will be dependent upon (normally linearly proportionalto) the oxygen concentration in a solution under investigation, byvirtue of a reaction involving the diffused oxygen at the interfacebetween the cathode and the membrane.

One arrangement in accordance with the invention will now be describedby way of example with reference to the accompanying drawing, which is adiagrammatic sectional view of part of a probe for measuring theconcentration of dissolved oxygen in an aqueous solution.

Referring to the drawing, the probe comprises a stainless steel tube 1having a diameter of about 0.1 inch, within one end of which is disposeda lead pellet 2, the pellet 2 being a force t within the tube 1 andhaving a projecting tapered nose. The pellet 2 has formed in it acentral hole of diameter 0.04 inch within which are disposed one end ofa silver wire 3 having a diameter of 0.02 inch and one end of aminiature cable 4 to which the wire 3 is joined, the cable 4 extendingthrough the tube 1 to provide an electrical connection for the wire 3;the wire 3 and cable 4 are secured in the hole in the pellet 2 byPatented Oct. 24, 1972 means of an epoxy resin 5, the end surface of thewire 3 being ush with the'free' end of the pellet 2.

In manufacture of the probe, when the structure so far described hasbeen fabricated, the nose of the pellet 2 is wetted with an aqueoussolution of ammonium sulphate, which is then allowed to dry to leave onthe surface of the lead minute crystals which subsequently serve as asource of hydroxyl io'ns. The end of the tube 1 in which the pellet 2 ismounted is then dipped into a solution of polyethylene in toluene .at atemperature of 115 C. and after removal from the solution the adherentmaterial is allowed to dry to form a membrane 6 of polyethylene whichcovers the projecting portion of the pellet 2 and the end surface of thewire 3.

The probe described above may be activated simply by exposing it tosteam for a time of the order of minutes, the exact time requireddepending on the thickness of the membrane 6. It also has the advantageof very low sensitivity to changes in the flow rate of a solution underinvestigation.

Where a probe is required solely for use in investigating solutions ofhigh and constant pH value, it is possible to simplify the arrangementdescribed above by omitting the ammonium sulphate crystals, since inthis case no internal source of hydroxyl ions will be required.

We claim:

1. An apparatus for use in measuring the concentration of dissolvedoxygen in an aqueous solution, said apparatus comprising:

a solid structure incorporating a pair of electrodes and means holdingsaid electrodes in spaced apart, insulated relationship; and

a homogeneous, water-absorbent, oxygen-permeable having first and secondfaces, said rst face being in direct contact with parts of said solidstructure, in-

cluding both of said electrodes, without the inclusion of any liquidmaterial between said electrodes and said lirst face of the membrane,and said second face being exposed for contact with a solution to beinvestigated.

I2. An apparatus according to claim 1, further comprising a source ofhydroxyl ions which will dilfuse into the membrane when it absorbswater.

3. An apparatus according to claim 1, in which the membrane is of apolymeric material.

4. An apparatus according to claim 3, in which the membrane is ofpolyethylene.

5. An apparatus according to claim 1, in which said electrodes arerespectively adapted to constitute the cathode and anode of a voltaiccell.

6. An apparatus according to claim 1, in which said membrane is adherentto said structure over the whole area of the membrane.

References Cited UNITED STATES PATENTS 3,088,905 5/1963 Glover 204-1953,098,813 7/ 1963 Beebey et al. 204-195 3,380,905 4/1968 Clark 204-1953,394,069 7/ 1968 Solomon 204-195 3,510,420 5/ 1970 Mills 204-195FOREIGN PATENTS 1,040,823 9/ 1966 Great Britain 204-195 TA-HSUNG TUNG,Primary Examiner U.S. Cl. X.R. 204-1 T v

